The present invention relates generally to methods and materials for the generation of hydrogen gas from solid hydrogen storage materials. In particular, the present invention relates to the generation of hydrogen gas by contacting water with micro-disperse particles of sodium borohydride in the presence of a catalyst, such as cobalt or ruthenium.
Hydrogen gas is used as a fuel for fuel cells, as a purge gas in chromatographs, and for combustors. Micro fuel cells, micro gas chromatographs (i.e., micro chem labs-on-a-chip), and micro-combustors all require a compact, high-density, controllable source of hydrogen gas. Gas cylinders are too heavy and bulky, while liquid hydrogen requires cyrogenic cooling. Metal hydride systems are limited to 1-3% hydrogen by weight; are endothermic (that is, as hydrogen is evolved, the container gets colder, which reduces the hydrogen vapor pressure); the hydrogen evolution rate is not controllable or adjustable (so that an oversized amount of hydride is necessary); and the metal hydrides are quite expensive.
Micropower is the key to making integrated microsystems. When a compact hydrogen source is developed to compliment a micro fuel cell, or feed a gas chromatograph, a total system package will be available for making power or purge gas, that currently is provided by large batteries or large gas cylinders.
Metal hydrogen complexes, such as sodium borohydride (NaBH4), zinc borohydride (ZnBH4), potassium borohydride (KBH4), calcium borohydride (CaBH4), lithium aluminum hydride (LiAlH4), sodium boron trimethoxy hydride (NaBH(OCH3)3), and so on, are attractive solid sources of hydrogen. When reacted with water, in the presence of a suitable catalyst, these metal hydrogen complexes can provide a hydrogen gas yield from 11-14% by weight (which is 5-6 times more hydrogen released per gram than for metal hydrides).
Sodium borohydride (also known as sodium tetrahydridoborate) is a particularly attractive solid source of hydrogen since its equivalent energy density is nearly equal to that of diesel fuel. It is commonly used in a variety of industrial processes (e.g., as a bleaching agent in paper and newsprint production and recycling). As an extremely powerful reducing agent, it is also used to reduce impurities in the chemicals processing industry, and to reduce metals from industrial waste streams and effluents (e.g., recovering copper from printed circuit board wastewater).
Sodium borohydride reacts exothermically with water in the presence of a catalyst (or when acidified) to produce hydrogen gas and sodium metaborate (i.e., Borax) according to the following reaction:NaBH4+2H2O→NaBO2+4H2+Heat(300 kJ) (catalyst)  (1)This reaction is particularly efficient at generating hydrogen gas, since the sodium borohydride supplies two of the hydrogen gas molecules (H2), and the water supplies the other two molecules, for a total of four molecules of H2. The reaction is exothermic; does not require the addition of heat or the use of high pressure to initiate; and can generate hydrogen even at 0 degrees C.
Solid sodium borohydride is a particularly attractive choice as a compact and high-density source of hydrogen fuel for fueling a Proton Exchange Membrane (PEM) hydrogen/oxygen fuel cell (i.e., PEM fuel cell).
Dry sodium borohydride is conventionally produced as a powder (i.e., particles) having particle sizes in the range of 100-600 microns. An anti-caking/flow agent additive is typically added immediately after removal of the borohydride powder from a vacuum dryer to promote free-flow of the powder (unless the dried powder is immediately compacted into a compacted product form). See U.S. Pat. No. 5,182,046 to Patton, et al.). Examples of compacted product forms include 25 mm diameter×6 mm pellets, 5×11×17 mm caplets, and granules. A typical anti-caking agent comprises 0.5% by weight of silica or magnesium carbonate nanoparticles. Here, the word “caking” means the uncontrolled agglomeration and/or aggregation of individual fuel particles into a larger mass (i.e., “cake”).
Alternatively, a fluidized bed dryer can be used instead of a vacuum dryer, which produces free-flowing particles of solid sodium borohydride without the need for using anti-caking or flow additives. See U.S. Pat. No. 6,231,825 to Colby, et al. This is because the fluidized bed process produces particles having a significantly larger average particle size (600 microns), as compared to the average size of vacuum dried particles (100-200 microns). Sodium borohydride is produced commercially by Morton and Eagle-Picher in the USA; by Finnish Chemicals (Nokia) in Finland; and Bayer in Germany.
Adding water to commercially available powders, granules, caplets, or pellets of solid sodium borohydride (in the presence of a suitable catalyst, such as cobalt or ruthenium) results in caking and scaling of the borohydride surface due to production of the reactant product sodium metaborate (NaBO2) in the form of a surface layer (i.e., crust or scale). As the layer of scale grows progressively thicker, the water has a progressively harder time penetrating through the metaborate crust to reach the unreacted NaBH4 fuel below, resulting in a decreased hydrogen production rate (it can even stop producing gas if the scale is thick enough). Note that this is a different phenomenon than the previous problem of “caking” caused by agglomeration of fine powders that occurs during production (that was described earlier).
The problem of scale/crust formation during hydrogen generation can be prevented by using a dilute aqueous solution of NaBH4. In this well-known approach, sodium borohydride powder is dissolved into water stabilized with 1-10% NaOH or KOH. The alkaline state of the solution prevents the dissolved sodium borohydride from decomposing and prematurely releasing hydrogen gas. The solubility limit of sodium borohydride in water at room temperature is 44 wt % (and decreases at lower temperatures). An example of a commercially available stabilized aqueous solution called VenPure® is available from Rohm and Haas, Inc. that comprises 12% NaBH4, 40% NaOH, and the balance water.
When using a dilute aqueous solution of NaBH4, hydrogen gas is generated by contacting the solution with a metal catalyst. For example, in U.S. Pat. No. 6,3587,488 (which is herein incorporated by reference), Suda discloses a method of generating hydrogen gas by adding small amounts of powdered catalysts, such as cobalt, nickel, or Mg2Ni (fluorinated or unfluorinated) to a stabilized, alkaline solution containing 10% sodium borohydride.
Likewise, the Hydrogen-on-Demand™ system from Millennium Cell, Inc. uses a liquid fuel source comprising a 20-30% aqueous solution of NaBH4 stabilized with 1-3% NaOH. In their system, a fuel pump and valves directs liquid fuel from a storage tank containing 20-30% sodium borohydride solution into a catalyst chamber (e.g., ruthenium sponge metal). Upon contacting the catalyst bed, the fuel solution generates hydrogen gas and sodium metaborate (in solution). The hydrogen gas and metaborate solution are separated in a second chamber, and the metaborate solution is stored as a waste product in a collection tank. The heat generated during the reaction is sufficient to vaporize some of the water present. As a result, the hydrogen gas is supplied at 100% relative humidity to the PEM fuel cell. The hydrogen gas can be optionally processed through a heat exchanger to achieve a specified level of humidity before being sent to the PEM fuel cell for consumption. This Hydrogen-on-Demand™ system has been successfully demonstrated to electrically power a Chrysler Town & Country Minivan.
Disadvantages of this approach include the relatively low energy density of the diluted liquid fuel (e.g., 10-30% NaBH4), which makes it only slightly better than metal hydrides. Additionally, a pump is required for circulating the liquid fuel, which causes a parasitic drain on the net power production from the fuel cell.
These disadvantages become particularly severe if liquid sodium borohydride is used for micro-sized PEM fuel cells, where the goal is to miniaturize every component in the system, while retaining high efficiency of fuel usage and power generation. The system requirements for a micro fuel cell or a micro gas chromatograph are considerably different than for a Minivan.
What is needed, therefore, is a material and system for generating hydrogen gas that utilizes solid sodium borohydride in a highly efficient manner that prevents caking and scaling from reducing the hydrogen production yield, and preferably, without using a fuel pump.
Such a device should provide 5-6 times more hydrogen than existing sources, with the additional possibilities of integration with silicon devices for sensing, control, and MEMS functions. This device should be able to be directly integrated with micro fuel cell designs to create a very compact micro power system. This device should also be scalable to larger systems needing larger amounts of hydrogen for higher power applications.
Against this background, the present invention was developed.